This application claims the benefit of Korean Application No. 2001-667, filed Jan. 5, 2001, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery with improved safety and reliability using a gel electrolytic solution.
2. Description of the Related Art
A lithium secondary battery generates electricity by lithium ions reciprocating between a cathode and an anode. The lithium secondary battery has a high energy density relative to the unit volume and voltage thereof as compared to a Nixe2x80x94Cd battery or a Nixe2x80x94H battery. In addition, the weight of the lithium secondary battery is approximately half that of the Nixe2x80x94Cd battery or the Nixe2x80x94H battery. Thus, the lithium secondary battery is suitably used for small size, light-weight, long-lasting electronic devices.
As described above, lithium secondary batteries have attracted particular attention because of their excellent characteristics, such as high voltage characteristics, improved charging/discharging cycle characteristics and environmentally benign characteristics and so on when compared to conventional Nixe2x80x94Cd batteries or Nixe2x80x94H batteries. However, since lithium secondary batteries can be highly explosive under certain conditions, safety is a critical issue for the practical use of the lithium secondary batteries.
Lithium secondary batteries are classified according to the type of electrolyte used. Specifically, lithium secondary batteries are classified into lithium ion batteries using a liquid electrolyte and lithium ion polymer batteries using a polymer electrolyte.
The lithium ion battery generally utilizes a cylindrical case or a rectangular case as a case for hermetically sealing the electrode assembly. However, recently, a greater attention has been paid to a method in which a pouch is used instead of the case because the use of the pouch increases energy density per unit weight or volume and allows attainment of small, lightweight batteries at low cost.
FIG. 1 is an exploded perspective view of a conventional lithium ion battery using a pouch as a case. Referring to FIG. 1, a lithium ion battery includes an electrode assembly 10 that includes a cathode 11, an anode 12 and a separator 13. A case 9 hermetically seals the electrode assembly 10. Here, the electrode assembly 10 is formed by interposing the separator 13 between the cathode 11 and the anode 12 and winding the structure. A cathode tap 15 and an anode tap 15a serve as electrical paths between the electrode assembly 10 and the exterior. The taps 15, 15a are drawn from the cathode 11 and the anode 12 to form electrode terminals 14 and 14a. 
FIG. 2 is an exploded perspective view illustrating a conventional lithium ion polymer battery. Referring to FIG. 2, a lithium ion polymer battery includes an electrode assembly 21 and a case 22 to hermetically seal the electrode assembly 21. Electrode terminals 24 and 24a serve as electrical paths to induce the current formed at the electrode assembly 21 to the exterior. The terminals 24, 24a are correspondingly connected to a cathode tap 23 and an anode tap 23a to then be exposed by a predetermined length outside the case 22.
As described above, in the lithium ion battery shown in FIG. 1 and the lithium ion polymer battery shown in FIG. 2, the electrode assemblies 10 and 21 are put into the cases 9 and 22, respectively, and an electrolyte solution is inserted thereinto. Only parts of the electrode terminals 14 and 14a and 24 and 24a are exposed to the exterior of the cases 9 and 22. Then, heat and pressure are applied to each resultant structure so that thermally adhesive materials at the edges of the upper and lower case parts cause the upper and lower case parts to adhere together to then be hermetically sealed, thereby completing the battery.
The lithium ion battery using the liquid electrolyte shown in FIG. 1 may experience leakage when the case 9 is damaged due to an external impact. Also, the electrode assemblies or pouches may be swollen due to evaporation of an organic solvent having a low boiling point.
To solve this problem, there have been proposed several methods of preparing batteries in which a solid or a gel-state electrolyte instead of a liquid electrolyte is coated on electrode plates, or a mixture of a liquid electrolyte and a polymerizable or crosslinkable monomer or polymer is cast on an electrode surface and hardened using ultraviolet rays, electron beams or heat as disclosed in U.S. Pat. Nos. 5,972,539, 5,279,910, 5,437,942 and 5,340,368. In practical application of the proposed methods, however, the preparation process is complicated and the battery performance is still not satisfactory.
To solve the above and other problems, it is an object of the present invention to provide a polymer electrolyte that effectively suppresses swelling due to an electrolyte solution and increases leakage resistance to improve the reliability and safety of the battery.
It is an another object of the present invention to provide a lithium secondary battery containing the polymer electrolyte.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The present invention provides a polymer electrolyte according to an embodiment of the invention that is prepared by polymerizing a composition including 0.1 to 90% by weight of a first compound represented by formula 1, a second compound represented by formula 2 or a mixture of the first and second compounds, 0.1 to 90% by weight of a third compound represented by formula 3, and 9.8 to 99.8% by weight of a nonaqueous organic solvent containing 0.5 to 2.0 M lithium salts, where:
Formula 1 is CH(R1)xe2x95x90C(R2)xe2x80x94C(xe2x95x90O)Oxe2x80x94R3-N(R4)(R5),
Formula 2 is CH(R1)xe2x95x90C(R2)xe2x80x94C(xe2x95x90O)Oxe2x80x94R3-CN, and
Formula 3 is Zxe2x80x94{xe2x80x94Yxe2x80x94Xxe2x80x94C(R2)xe2x95x90CH(R1)}n.
According to another embodiment of the invention, R1 and R2 can be the same or different and are selected independently from the group consisting of hydrogen, C1 to C10 alkyl, fluorinated C1 to C10 alkyl, C6 to C14 aryl, and fluorinated C6 to C14 aryl.
According to yet another embodiment of the invention, R3 is selected from the group consisting of 
According to still another embodiment of the invention, R4 and R5 are selected from the group consisting of 
in which R4 and R5 are the same or different.
According to still yet another embodiment of the invention, X is selected from the group consisting of 
and Y is selected from the group consisting of 
R6 is hydrogen, methyl, ethyl, propyl or butyl group, m is an integer in the range of from 0 to 10 inclusive, n is an integer in the range of from 1 to 6 inclusive, and Z has the following structure according to the n value:
Z is H, or C1 to C12 alkyl group when n=1;
Z is selected from the group consisting of 
when n=2.
According to a further embodiment of the invention, Z is selected from the group consisting of 
when n=3.
According to a still further embodiment of the invention, Z is selected from the group consisting of 
when n=4.
According to a yet further embodiment of the invention, Z is selected from the group consisting of 
when n=5 or 6.
According to a yet still further embodiment of the invention, the polymerization may be selected from the group consisting of thermal polymerization, electron beam polymerization and UV polymerization, the polymerization temperature for the thermal polymerization is in the range of 20 to 100xc2x0 C., and the wavelength of light for the UV polymerization is in the range of 200 to 400 nm.
According to an additional embodiment of the invention, benzophenone compounds such as benzophenone or substituted benzophenone are used as a polymerization initiator for the UV polymerization.
According to a yet additional embodiment of the invention, diacyl peroxides such as benzoyl peroxide, acetyl peroxide or lauroyl peroxide, azo compounds such as azobisisobutylonitrile (AIBN), azobis(2,4-dimethyl valeronitrile), azobis(cyclohexanecarbonitrile), or peroxy ester compounds such as t-butyl peroxy ester, t-amyl peroxybenzoate, peroxy carbonate compounds such as t-butyl peroxy isopropyl carbonate, t-butyl peroxy 2-ethylhexyl carbonate are used as a polymerization initiator for the thermal polymerization.
According to a still additional embodiment of the invention, no polymerization initiator is used for the electron beam polymerization.
According to yet still additional embodiment of the invention, when the UV polymerization or thermal polymerization is used, the composition further includes 0.1 to 10% by weight of the polymerization initiator, based on the total weight of the compounds represented by formulas 1, 2, and 3.
According to an additional embodiment of the present invention, a lithium secondary battery includes a cathode and an anode capable of accepting/releasing lithium ions, and a polymer electrolyte prepared by coating the composition of the present invention on one of the cathode and the anode and polymerizing the coated composition.
According to a yet additional embodiment of the present invention, the lithium secondary battery further includes a separator interposed between the cathode and the anode.
In accordance with a yet additional embodiment of the present invention, a lithium secondary battery includes a polymer electrolyte prepared by polymerizing the composition and disposed in a case incorporating a battery assembly having a cathode and an anode capable of accepting/releasing lithium ions, and a porous separator interposed between the cathode and anode.